A typical recording medium used as a secondary storage device in a computer system is a hard disk drive (HDD: Hard Disk Drive). In recent years, a solid state drive (SSD: Solid State Drive) based on a semiconductor memory also is used. The HDD and the SSD have different characteristics in terms of cost per unit capacity, performance and power consumption. In general, the cost per unit capacity of the HDD is lower than that of the SSD, and the performance and power consumption of the SSD are superior to those of the HDD (Non Patent Literature 1: Graefe, G., “The Five-minute Rule 20 Years Later: and How Flash Memory Changes the Rules”, Queue 6, 4 (July 2008), 40-52, refer to http://doi.acm.org/10.1145/1413254.1413264).
Moreover, the performance of the SSD and the HDD varies greatly depending on products. For example, there are two types for the SSD; a single level cell (SLC) with high performance and a multi-level cell (MLC) with low performance. The performance of the HDD strongly depends on a rotational speed and a seek time of a disk. In general, products with higher performance tend to be higher in the cost or the cost per unit capacity. More detailed performance of the recording media is described in Non Patent Literature 2 (Dumitru, D., “Understanding Flash SSD Performance”, http://www.storagesearch.com/easyco-flashperformance-art.pdf).
In this manner, there are various types of recording media and those characteristics are different from each other. Existing techniques related to usage of the various types of recording media will be described below.
Patent Literature 1 (Japanese Patent Publication JP-2006-18591) discloses a memory card that has a high-capacity flash memory with a large write data unit and a low-capacity flash memory with a small write data unit. At writing of a data whose size is less than a page size of the high-capacity flash memory, the said data is written to the low-capacity flash memory.
Patent Literature 2 (Japanese Patent Publication JP-2008-242503) discloses a storage device that has a high-speed SLC flash memory and a low-speed but high-capacity MLC flash memory. A frequently-accessed data is written to the high-speed SLC flash memory.
Patent Literature 3 (Japanese Patent Publication JP-2008-310793) discloses a memory device that has a high-speed SLC flash memory and a low-speed but high-capacity MLC flash memory. The high-speed SLC flash memory is allocated to a head space of a memory region, in which data management information of a file system is stored.
Patent Literature 4 (Japanese Patent Publication JP-2007-193440) discloses a memory device that has an HDD and a nonvolatile cache memory. When receiving a command instructing data write, a state determination unit checks a rotational state of the HDD. If the HDD is in a rotating state, a data is written to the HDD. If the HDD is not in the rotating state, a data is written to the nonvolatile cache memory.
Patent Literature 5 (Japanese Patent Publication JP-2007-293440) discloses a memory device that has an HDD and a flash memory. In a case of a predetermined command for write through that instructs writing of a data to a plurality of media, the data is written to both of the HDD and the flash memory.
Patent Literature 6 (Japanese Patent Publication JP-2008-46964) discloses an information recording device that has an HDD, a flash memory and an SDRAM operating higher than the flash memory. The flash memory and SDRAM both are used as cache memories for the HDD. If there is currently no free space of a size of a write data but expected to be formed before long, the information recording device determines which is faster to write the said write data to the SDRAM or to the flash memory. Then, the said write data is written to the one determined to be faster.
Patent Literature 7 (Japanese Patent Publication JP-2000-3309) discloses a memory device that has a cache control unit, an access type determination unit and a buffer configuration control unit: When receiving a write access from an upper-layer device, the cache control unit loads a write data into a data buffer region of a memory, notifies the upper-layer device of successful completion, and then writes the write data loaded in the data buffer region to a recording medium. The access type determination unit analyzes whether the write access from a host is a sequential access or a random access. The buffer configuration control unit selects, depending on the access type, a data buffer configuration with an optimum number of sections and performs an cache operation.
Patent Literature 8 (Japanese Patent Publication JP-2007-65743) discloses a mobile information terminal that has an HDD and a buffer memory. A memory region of the buffer memory is clearly divided into a first region for use in storing sequential data such as audio data and a second region for use in storing random access data such as address files. The random access data is always recorded on the second region.
As another related technique, Patent Literature 9 (Japanese Patent Publication JP-2000-322315) discloses a method of managing data. According to the method, data objects in object heap are organized based on an access relationship between the data objects (e.g. based on temporal accessibility or relative frequency of accesses). As a result, data objects that are closely and successively accessed are arranged in the same page and thus are more likely to be arranged in the same cache line.